This invention relates to testing of vehicles, and more particularly to testing dampers of automobiles and the like.
An automotive suspension is meant to provide both safety and comfort for the occupants. When a vehicle hits a bump in the pavement, the shock is taken up by various components of the vehicle. For example, the tire deforms as the suspension displaces, while most of the energy is stored in the suspension springs. The springs release this energy as a damped oscillation. Shock absorbers and struts (hereinafter referred to collectively as dampers) in motor vehicles serve to damp oscillations of the vehicle chassis resulting from travel over uneven surfaces and to reduce the magnitude of spring deflections in response to large bumps.
More specifically, a damper is a hydraulic mechanism positioned between sprung and unsprung masses to dissipate kinetic energy put into the system by bumps. Dampers provide desired ride characteristics, but also play a key role in keeping good tire-to-road contact essential for handling and safety. The dampers are probably the most versatile members of the group of components that affect ride and handling. The dampers control vibration and improve handling and load control. Without dampers, a car would go out of control at just twenty miles per hour and braking distance would increase. One faulty damper can cause unbalanced damping in suspension. Weak dampers allow the vehicle to continue to oscillate three or more times after the disturbance, causing an undesirable condition known as "float."
Some signs of a worn damper are (1) riders feeling carsick or tired, (2) excessive wear on the tires due to bumpy contact with the road, (3) vibration of steering, suspension and body parts, (4) car roll on turns, (5) nose-dive when braking, (6) veer in crosswinds, (7) headlights pitch up and down when driving over uneven surfaces, and (8) oil leakage from the dampers.
Dampers can be divided in the compression stage and in the rebound stage to either "firm" or "soft." For ride comfort, both compression and rebound stages should be "soft," but vertical body motion velocity could considerably exceed desirable limits. The damper rebound stage should be "firm" and compression should be "soft" to minimize vertical body motion velocity. But if vertical body velocity is low, usually on a smooth road, the damper compression stage should be "firm" and the rebound stage "soft", which should maximize adhesion. For ride safety, especially at wheel hop resonance, both the compression and rebound stages should be "firm."
Many car manufacturers try to compromise these factors for off-highway (rougher roads) driving using much more damping in rebound (two to six times) than in compression. This minimizes body motion velocity, but does not always maximize ride comfort. It also reduces the safety of the ride, especially at higher speeds.
Although the damper is a critical element in the safe operation of a vehicle, previously it has not been easily tested conclusively without completely removing it from the vehicle. Because of this, various methods have been proposed for testing the quality of a damper. One of these methods is for the mechanic to physically oscillate the vehicle by hand and observe the resulting oscillations of the vehicle. This method is less than satisfactory, since its results are not quantitative. Another method involves dropping the vehicle from a predetermined height to generate oscillation of the vehicle. This second method is superior to the first, but still could be improved since it reflects the response of the damper at a single frequency of excitation. Neither method provides testing over as wide a range of frequencies as might be desired, since both are basically low frequency tests.
One measure of the roadworthiness of a vehicle can be given in terms of the minimum amount of traction that the wheels will provide on uneven pavement. This number is called the "adhesion." This minimum traction allows the vehicle to maneuver despite bumps and other irregularities in the pavement. A method of testing adhesion has been proposed by the European Shock Absorber Manufacturers Association (EUSAMA). This method, set out in Recommendation TS-02-76 issued by EUSAMA includes the following requirements: (1) the vertical static contact force between the tire and the support member on which the tire associated with the damper to be tested is disposed is measured; (2) the supporting member is given a sinusoidal excitation to bring the vehicle into vibration; (3) adhesion is defined as the ratio of the minimum dynamic contact force measured on the supporting member at the wheel resonance frequency to the static contact force, expressed as a percentage; and (4) the proposed minimum frequency for exciting the supporting member is 24 Hz. Using the EUSAMA standards, when a test result of less than 20% adhesion is obtained, the damper is unsatisfactory; from 20% to 40% the damper is fair; above 40% adhesion, it is good, and over 60%, it is excellent.
The apparatus for performing the EUSAMA damper test includes a supporting member for the tire, and a drive motor with eccentric and spring for driving the supporting member. The frequency of vibration with this apparatus is increased into the range beyond the resonance frequency of the chassis (vehicle suspension). Then, the excitation frequency is gradually reduced. As the frequency is reduced, the frequency of vibration passes through the natural resonance or "wheel-hop" frequency.
In fact, there are a number of vibrational frequencies associated with a vehicle. Any vehicle is made up of a number of different components (rigid body, suspension, wheel, chassis, body panels, steering wheel, engine, etc.) with different vibrational behaviors. Vibration of the rigid bodies occurs at low frequencies, such as 0.5 to 5 hertz. Vibration of the suspension, on the other hand, occurs at frequencies of 5 hertz and above. In this regard, it should be noted that the primary difference between independent and linked suspensions is that while the independent suspension can be expected to have a single wheel hop resonance in the ten to twenty hertz frequency range, the linked suspension exhibits a pair of resonances. Both the suspension spring and the rubber suspension bushings play an important part in the actual vibrational levels transmitted across the suspension and entering the vehicle chassis at higher frequencies.
With respect to vibration of the vehicle chassis, for a fully dressed vehicle, the resonant frequencies are lower than for the free chassis due to the mass. The frequency range of vibration usually begins around ten hertz and can extend out through several hundred hertz. A vehicle chassis can possess a large number of different vibrational modes, some of which may have resonant frequencies in the range where wheel hop and other low frequency problems can occur.
It is known that low frequencies in the 0.5 to 30 Hz range are felt tactually by the human body, while vibrations in the range of from 20 Hz on up may be heard. Human beings are most sensitive to tactile vibrations in the range of four to eight hertz, but the sensitivity falls off rapidly as the frequency rises. Moreover, the greater the amplitude of the vibration, the greater the sensitivity of the person to that particular frequency of vibration.
Although the EUSAMA system is an improvement over prior methods, it can also be improved. The testing in the EUSAMA system is actually a test of the entire suspension system, not a test of the adequacy of the damper alone, and even then only one parameter, adhesion, is considered.
Ideally a test of a vehicle suspension should examine not only adhesion, but also actual adequacy of the damper under test, and the balance of adhesion from side to side of the vehicle. Prior suspension testers are not believed to test these various aspects of the acceptability of the suspension.